The present embodiments relate to determining a measuring sequence for a magnetic resonance system based on at least one intra-repetition-interval time parameter.
In a magnetic resonance system, the body to be examined may be exposed to a relatively high basic magnetic field of 3 or 7 tesla, for example, with the aid of a basic field magnetic system. A magnetic field gradient is also applied with the aid of a gradient system. High-frequency excitation signals (HF signals) are emitted by suitable antenna devices via a high-frequency transmission system, which is intended to cause the nuclear spins of specific atoms that have been excited in a resonant manner by this high-frequency field to be tilted by a defined flip angle relative to the magnetic field lines of the basic magnetic field. This high-frequency excitation and/or the resulting flip angle distribution are also referred to below as nuclear magnetization or “magnetization.” During relaxation of the nuclear spins, high-frequency signals (e.g., magnetic resonance signals) are emitted, received by suitable receiver antennas and then processed further. The desired image data may be reconstructed from the raw data acquired in this way.
As explained above, for a specific measurement, a specific measuring sequence including a sequence of high-frequency pulses (e.g., excitation pulses, refocusing pulses and gradient pulses for transmission in a suitably coordinated manner in different spatial directions and suitably adapted readout windows, during which the induced magnetic resonance signals are acquired) is emitted. A factor for the imaging is the timing within the sequence (e.g., which pulses follow each other in which time intervals). A plurality of control parameters may be defined in a measuring protocol that is compiled in advance and may be called up for a specific measurement from a memory, for example, and optionally changed by the operator, who is able to specify additional control parameters such as, for example, a specific slice gap in a stack of slices to be measured, a slice thickness, etc. All these control parameters are used as the basis for the calculation of a measuring sequence.
Therefore, during a measuring sequence of this kind, the magnetic gradient coils are frequently switched over. Both the maximum amplitude of the gradient pulses and the rise time of the gradient current (e.g., the rise in the edges of the gradient pulses, termed the “slew rate”) are important gradient-pulse parameters that affect the efficiency of a magnetic resonance tomography device. For example, numerous imaging sequences use high slew rates and high gradient-pulse amplitudes. However, the induction of eddy currents by the gradient pulses into surrounding metallic surfaces (e.g., the high-frequency screen of the magnetic resonance tomography device or even into the body of a patient or test subject) is unfavorably linked to the slew rate. For example, the higher the slew rate, the higher the eddy currents. The eddy currents in the body of the patient or test subject may result in artifacts and to peripheral nerve stimulation (PNS). Eddy currents with other components of the magnetic resonance tomography device (e.g., the high-frequency screen) are one reason for the known noise phenomena during the gradient switching. Similarly, noise development is intensified by higher gradient amplitudes since these also result in higher forces within the magnetic resonance tomography device. A higher slew rate and higher gradient amplitudes also result in higher energy consumption and place higher requirements on the hardware.
For example, to reduce noise emissions, there have already been various proposals with respect to the design of the hardware such as, for example, encapsulation or vacuum sealing of the gradient coils. Another possibility includes providing, as early as the calculation of the measuring sequences, that the slew rate and the gradient amplitude are limited to the greatest degree possible. Devices are thus found offering different “gradient modes.” The operator may choose between a normal mode and a particularly quiet gradient mode, for example. In the quiet gradient mode, a maximum permissible slew rate for the gradient pulses is set to a lower value, which results in the measurement being quieter than in the normal mode. However, unfavorably, this may not only result in a longer measuring time overall, but also causes a reduction in the image quality (e.g., the contrast and/or the resolution). Hence, a limitation of the global maximum slew rate of this kind entails striking a compromise between the reduction in the sound level, the measuring time and the image quality.